The basic steps in the manufacture of sheet glass, e.g., sheet glass for use as substrates for electronic displays, such as liquid crystal displays (LCDs), include: (1) melting raw materials, (2) fining (refining) the melt to remove gaseous inclusions, (3) stirring the fined molten glass to achieve chemical and thermal homogeneity, (4) thermally conditioning the homogenized glass to reduce its temperature and thus increase its viscosity, (5) forming the cooled molten glass into a glass ribbon, and (6) separating individual glass sheets from the glass ribbon. In the case of a downdraw fusion process, the glass ribbon is formed using a forming body known as an “isopipe,” while in a float process, a molten tin bath is used for this purpose. Other methods as are known in the glass making arts may also be used.
High temperatures are needed to successfully fine molten glass since the rate of rise of gaseous bubbles through molten glass varies inversely with the viscosity of the glass. That is, the lower the viscosity, the faster the rate of rise. Glass viscosity varies inversely with temperature, accordingly, the higher the temperature, the lower the viscosity. Because molten glass is in the apparatus used for fining for only a limited amount of time, achieving a rapid rise of bubbles through the melt is of great importance. Hence, the finer is normally operated at as high a temperature as possible and therefore the molten glass is at a low viscosity. However, to form molten glass into a ribbon requires viscosities much higher than those used during fining. Hence, the need to thermally condition (cool) the molten glass between fining and forming.
Historically, thermal conditioning has been performed by passing the molten glass through a conduit having a circular cross-section. The conduit has been surrounded by ceramic material and supported by a metal frame, and the rate of heat loss from the molten glass has been controlled through the use of direct or indirect heating so as to avoid introducing substantial thermal and flow inhomogeneities into the glass as a result of the cooling process. Because of the high temperature of the molten glass and the need to avoid contamination of the molten glass, the wall of the conduit is often formed from a precious metal, for example, a platinum group metal.
Among the valuable characteristics of platinum-containing materials is their ability to generate heat when conducting electricity. As a result, molten glass flowing through, or held in, a platinum-containing vessel can be heated by passing electrical current between one or more locations along the length of the vessel's glass-contacting wall. Such heating is known in the art as “direct heating” or “direct resistance heating,” the term used herein. In this usage, “direct” denotes heating from the vessel itself, rather than through externally applied indirect resistance or flame heating.
A major challenge in direct resistance heating is the introduction and removal of the electric current from the vessel's wall. This is not only an electrical problem, but is also a thermal problem since the conduction path can lead to unbalanced current densities that create hot spots in the conduction path. These hot spots can lead to premature material failure, such as through accelerated oxidation of the metals involved or by reaching the melting point of the metal.
One way of introducing current into a vessel's wall is through the use of an electrically-conductive metal flange. Examples of such flanges can be found, for example, in U.S. Pat. Nos. 6,076,375 and 7,013,677. The present invention is concerned with flanges used to introduce current into a platinum-containing vessel wall and, in particular, ensuring a uniform current density within the flange and the vessel carrying the molten glass.